In recent years, there have been rapidly proceeded the research and development for a high-density optical disc system, which is capable of recording and/or reproducing information (hereinafter, “recording and/or reproducing” will be expressed by using following wording “recording/reproducing”) by using a blue-violet laser diode having wavelength of about 400 nm. For example, there is provided an optical disc, what is called a “Blu-ray Disc” (hereinafter it will be called BD), used for recording/reproducing information based on the standard that NA (Numerical Aperture) is 0.85 and wavelength of a light source is equal to 405 nm. As for BD, information of 23-27 GB per a layer can be recorded on the optical disc having a diameter of 12 cm, which is the same size as a DVD which is used based on the standard that NA is 0.6 and wavelength of a light source is 650 nm, and whose recording capacity is 4.7 GB. There is also provided an optical disc, what is called a “HD DVD” (hereinafter it will be called HD), used for recording/reproducing information based on the standard that NA (Numerical Aperture) is 0.65 and wavelength of a light source is equal to 405 nm. As for HD, information of 15-20 GB per a layer can be recorded on the optical disc having a diameter of 12 cm. These discs are named “a high density optical disc”.
On the other hand, it is sometimes considered that a product, such as an optical disc player and a recorder (hereinafter it will be called an optical disc player/recorder), which is capable of only recording/reproducing information for a high-density optical disc is worthless. Taking account of a fact that, at present, DVDs and CDs (Compact Disc), onto which various kinds of information have been recorded, are on the market, the value of the product as a high-density optical disc player/recorder is increased by, for example, enabling to appropriately record/reproduce information additionally for DVDs and CDs, which users possess. From these backgrounds, the optical pickup apparatus installed in the high-density optical disc player/recorder is required to be capable of appropriately recording/reproducing information not only for a high-density optical disc but also a DVD and a CD.
As a method for appropriately recording/reproducing information for any disc of a high-density optical disc, DVD and further CD while keeping the compatibility, it is feasible that a method of selectively switching the optical systems corresponding to the recording density of discs: the high-density optical disc, the DVD and further the CD, for which information is recorded/reproduced. However, since a plurality of optical systems is required for the method, it is disadvantageous for the minimization of the product and which increases the cost of the product.
So, it is preferable to commonly share the optical system for the high-density optical disc and the optical system for the DVD and CD as much as possible in an optical pickup apparatus having compatibility in order to simplify the structure, to decrease the cost of the optical pickup apparatus and to decrease the number of optical parts structuring the optical pickup apparatus as much as possible. Further, to commonly share the objective lens, which is placed so as to be opposing to the optical disc, has advantages from the viewpoints of simplifying the structure and decreasing the cost of the optical pickup apparatus.
However, when realizing the compatibility in the optical pickup apparatus by applying a common objective lens, it requires an idea for forming a converged light spot whose aberration is well corrected on the information recording medium, because of wavelength difference between the light source wavelengths used for respective optical discs.
An embodiment of the aberration correction is to place a coupling lens, which is shiftable in the optical axis direction, between the light source and the objective lens and to shift the coupling lens in the optical axis direction to change the divergent degree of a light flux entering into the objective lens corresponding to the optical disc to be used. However, in order to shift the coupling lens in the optical axis direction, an actuator for shifting the coupling lens is required. Since in order to secure the setting space and the shifting space of the coupling lens, there is a problem that the size of the optical pickup apparatus becomes large and the cost increases. In the case of that a liquid crystal display is inserted between the light source and the objective lens, the same problem that the cost increases, occurs.
With respect to anther embodiment of the aberration correction, the following idea is proposed in order to realize the compatibility. A diffractive structure with wavelength selectability is provided on the optical surface of the objective lens to generate diffracted light fluxes with different orders corresponding to the three types of light fluxes passing through the objective lens. According to the structure, since the coupling lens is placed stably, the actuator is not necessary. However, there is a problem that the diffractive structure for generating the diffracted light fluxes with different orders lowers the light utilization factor of the specific type of light flux.
Japanese Patent Application Open to Public Inspection (JP-A) 2005-209250 disclosed the method for realizing the compatibility across the three types of different optical discs by differentiating the image magnification when using a high-density optical disc, the image magnification when using a DVD, and the image magnification when using a CD.
However, according to the technologies disclosed in JP-A 2005-209250, since a finite converging light flux enters the objective lens when using a high density optical disc, the tracking characteristics of the optical pickup apparatus becomes worse and there is a possibility that reading errors occur when the amount of tracking shift is large. Accordingly, the tracking characteristics need to be improved. However, when trying to improve the tracking characteristics, the amount of the offence against the sine condition is hard to be suppressed. Particularly, when shortening the focal length of the objective lens and guiding a finite light flux into the objective lens in order to make the optical pickup apparatus thin, there is a tendency that coma aberration when tracking tends to notably occur.